Micropitting is a term introduced initially by the gear industry to describe tiny surface spalls and cracks, sometimes appearing on the surface of rolling/sliding contacts. The standard ISO 15243 [1] on Rolling Bearings—Damages and Failures—refers to this damage or failure mode as “surface distress” or “surface initiated fatigue”, which is: The failure of the rolling contact metal surface asperities under a reduced lubrication regime and a certain percentage of sliding motion causing the formation of (1) burnished areas (glazed; grey stained), (2) asperity microcracks, (3) asperity microspalls.
In many industrial bearing applications, power density has increased substantially due to the need for higher efficiency (weight and cost reduction by downsizing). With the increasing severity of the working conditions (e.g. heavier loads in combination with higher temperatures, thinner oil films and/or boundary lubrication conditions) machine components can suffer from surface initiated fatigue (or micropitting). In rolling element bearings, micropitting is not necessarily a primary failure mode, but it can facilitate/accelerate the appearance of other failure modes like indentations, surface initiated spalling and seizure.
Thus, micropitting is one of the mechanisms responsible for life-limiting bearing wear. One approach to mitigating the effects of micropitting is to ensure that the rolling contact surfaces in a bearing are always separated by a lubricant film of sufficient thickness. Especially when grease lubrication is employed, however, the bearing will at times operate under a boundary or mixed lubrication condition. In short, adequate film thickness cannot always be ensured.
A second approach to combating micropitting and improving the wear and fatigue life of bearings is to employ surface engineering techniques, which alter the chemical and/or topographical properties of the rolling contact surfaces. An example of such a technique is disclosed in U.S. Pat. No. 5,503,481, issued to The Timken Company. The patent describes a process for providing an isotropic finish on rolling contact surfaces of a bearing. After a machining operation such as grinding, which creates an oriented roughness profile on the rolling surface of a bearing component, the bearing component is immersed in a chemical solution which reacts with the bearing steel to form a protective coating. The chemical solution further comprises abrading elements which, via agitation, remove the protective coating on the upper elevations of the roughness profile, enabling a further chemical reaction with the underlying steel. The height of the roughness profile therefore diminishes, to leave shallow pits with no particular orientation (i.e. isotropic). Finally, in the absence of the chemical solution, the protective coating is removed.
The resulting finish is often referred to as a superfinish, and has been shown to significantly improve resistance to the mechanisms of bearing wear, including micropitting. Bearings with a superfinish are, however, relatively expensive. Consequently, there is room for improvement in terms of providing a bearing with increased resistance to micropitting, which can be manufactured in a straightforward and economical manner.